Timing light out at full advance

Searching for the tune on my chopper with the 77 CB750 motor, I'm seeing an interesting phenomena that internet searches aren't explaining. When checking my timing with a timing light, I am getting good readings on 1/4. Timing is spot on at idle and at full advance. When I check 2/3, however, the light is a bit intermittent at idle, then completely goes out at full advance. This suggest the plugs are on 2/3 are not firing at all at full advance. Primary and secondary resistance on the coils checks out just fine and the ignition is static timed. Good power to coils. I am getting fouling on the plugs, so my conclusion is that it's the fouling that is causing this issue. Basically, I am guessing that at full throttle it's flooding the chambers and my really dark, basically fouled plugs are cutting out. But when I let off the throttle, I get light again, so I am not 100% confident my conclusion is correct. Anyone else experience this that may have insight?

Might also check the 12V supply to that coil at the different speeds. Make sure a poor connection isn't dragging it down as more current is needed at higher RPM.

eric narrow the plug up to about 23 thou and try it again once you verify coil voltage

on old bikes like these and the dohc i do the coil relay mod run a fuse off teh battery to relay use the harness 12 that went to the coils to trigger the relay

use the out put of the relay to direct power the coils

matt has a plug and play kit for this

also make sure you do not have play in theadvance or points cam and that bounce is not closing or opening the gap up enoung

that ypu are losing trigger to the 2-3 coil

take a non contact infa red thermometer measure all the pipe and see what i happening

also swap the coils and see if the issue moves from 2-3 to 1-4

+ 1 to that. Plus if the mix is too rich, it will not fire consistently. I'd start with the relay to ensure maximum number of electrons make it to the coils and a smaller gap to make it easier for them to jump to the other side as first steps. It may also be that the coils is breaking up or that side points are not making as good contact or the condensor is leaking slightly. A DYNA S set up would eliminate some of those variables.

also check high tension wires and plug caps for continuity and leaks

cxman said:

eric narrow the plug up to about 23 thou and try it again once you verify coil voltage

on old bikes like these and the dohc i do the coil relay mod run a fuse off teh battery to relay use the harness 12 that went to the coils to trigger the relay

use the out put of the relay to direct power the coils

matt has a plug and play kit for this

also make sure you do not have play in theadvance or points cam and that bounce is not closing or opening the gap up enoung

that ypu are losing trigger to the 2-3 coil

take a non contact infa red thermometer measure all the pipe and see what i happening

also swap the coils and see if the issue moves from 2-3 to 1-4

Click to expand...

Thanks, Murray. I'll do all of this and report. One thing I do want to point out is that the coils are in fact wired similar to your suggestion. I have verified 12.2v at the coils with ignition on. I'll get numbers for idle and throttle. One thing this is verifying is why the bike falls flat on throttle and hits a wall at full advance. It's clearly running on two cylinders when it's opened up, assuming this is what is happening on the road. I'll have to go back and check my notes, but I was getting a similar effect when I had the Dyna S installed. That's why I went back to points, plus the power consumption people claim with the Dyna S. Could it suggest the possibility that if it does this with both the Dyna S and with points than we may have an issue with the advance mechanism?

I also ordered a set of stock replacement coils to replace these green Dyna coils, in case that is the issue.

TheCoffeeGuy said:

also check high tension wires and plug caps for continuity and leaks

Click to expand...

I've run 3 different wire sets and I just installed a fresh set this weekend. With high tension wires, you don't so much get a continuity reading as much as you get a resistance reading. Still requires continuity, but I installed MDS low resistance wires this time around (35 ohms/ft) and added shrink on top of the caps. These are zero resistance caps. I may run some NGK resistor caps when they get here in my mail.

I had thought that the dyna coils were 3ohm and the coils used for points were 5 ohms.

canyoncarver said:

I had thought that the dyna coils were 3ohm and the coils used for points were 5 ohms.

Click to expand...

Yes. But it did this with the same set of coils and both Dyna S and points. At least I suspect it did. The same thing was happening while riding, etc. But I didn't confirm the advance with a light when running the Dyna, since you set the Dyna by manually advancing the mechanism and setting it at the advanc marks.

If the same thing happens with DYNA S trigger as with points, we can assume that part of the circuit is OK. With DYNA coils, 3 ohm are better for race bikes and 5 ohm for the street.

If you had a pair of new DYNA coils, you could sap them side to side and see if the problem moved. That might indicate a bad coil.

But if the same situation existed with DYNA coils and stock coils, that's less likely to be the issue.

That doesn't leave a lot except perhaps a bad wire from points/ DYNA S to coils.

I pulled the points ignition and installed the newer style Dyna S, which I bought a while back to replace the older style Dyna, but never installed. I didn't change anything in the wiring, and I kept the same coils. All I did was plug the appropriate Dyna trigger wires into the yellow and blue wires for the coils and ran a wire for power to the Dyna. I cleaned and lubed the advancer unit, as well. There is a small hole in the shaft on the advancer where the cam goes that had what looked like a burr. The best conclusion I can come up with is that maybe it had a bad condenser or that burr somehow effected the advance. I cannot explain why the other Dyna produced similar results, unless perhaps it had it's own issues and it created some strange coincidence. Regardless, I set a static time, started it up, ran it with a timing light, adjusted the plate and adjusted until it was right on the money. All good.

Something of note: There is a pin on the back of the advancer that sits in a hole at the end of the crank. This pin does not have a tight fit in that hole, which means the advancer has some play in it before things are tightened down. The rod that the cam slides over is also not a tight fit through the cam. It also has some play until you tighten down the mounting screw. I need to figure out if that is normal, or if that could also be a possible culprit for my issues.

Something else I cannot explain is that according to the Dyna S instructions, I have mine hooked up backwards. 1-4 and 2-3 are installed opposite. But, according to my timing light, mine are installed exactly right. My only guess is that I have a single magnet cam (which should be double) installed opposite on the advancer shaft making it 180° out. Regardless, it runs.

I can't remember for sure but I think the DS1-1 has a single magnet and yes, the rotor can be mounted 180 degrees out on the ATU.

With respect to slop in the system and on the crankshaft roll pin, you are right that it changes where it's timed, but the slop is the same on both sides, so what impacts one pair should be the same for the other pair.

The question is why does it miss/die out at full advance and maybe the issue is not so much the advance but the revs. Check teh air gap on both pickups and see if it's OK. It may be that the points plate is offset to one side so the air gap is too large on one.

You could have a defective module but the chances of a dead one in two sets is pretty small.

Another thought just struck me. Double ended coils require that both plugs are OK and firing. If one is dead or the cap is defective, neither on that coils should fire. Try swapping the plugs and caps side to side so to speak.

teaser for the record and diagnosis

my carb setup fires cylinder 1 and 2 the other 3 and 4

so eric had a issue of 2 and 3 so i know it was ignition

Good feedback. Thanks.